Many types of devices may be used to anchor an offshore structure to the seafloor. Cement bases or blocks, clump weights, and standard cylindrical piles are just a few examples of how to accomplish siting in some types of offshore structures. However, these anchoring methods and devices may not be adequate or exacting enough for platforms with a higher positive buoyancy such as a Tension Leg Platform (TLP). One common technique that is used to anchor a TLP connects the leg tendon directly to a hollow cylindrical anchor piling. The bottom of the pile is cut at a 45.degree. angle then stabbed into a template on the seafloor and driven with a hammering device until it may no longer continue. This termination is based on the structure in the underlying seafloor and is called the depth of refusal. Since the friction on the pile is what resists uplift loads, driving to a refusal depth is done to obtain as much capacity as possible for a given piling. Safety improvements and cost reductions can be accomplished if the tendons connect directly to the piling. To do this, it is essential that the piling be driven exactly to a predetermined grade in order that the tendons that connect the buoyant structure to the seafloor be of equal length and tension. With variations in the soil properties between piling locations it is unlikely that all the piling may be driven to a preset depth thus presenting the problems mentioned above. Furthermore, when a pile cannot be driven any further it must be cut off so that all the tendon lengths will be equal. This is also undesirable because of the necessity of having machined surfaces that are compatible with tendon coupling.
An existing alternative method involves the use of drilled and grouted piling. Here, a template is used to drill an oversized hole, i.e., one larger than the piling. Once the hole is completed, the pile is lowered into the hole and cemented. This may be done in any number of ways. For example, cement may be forced in the annular space between the pile and the soil from the bottom up or it may be squeezed through holes in the pile, again in stages from the bottom up. The force that is used to resist uplift loads is friction between surfaces, i.e., the soil/cement interface and the cement/steel pile interface. The disadvantages of this method are the uncertainties in installation techniques and the resulting piling capacity. For example, placement of the grout may be difficult, the hole for the piling may not be straight, the mud around the piling itself may tend to cake and interfere with the entire process, the grout may flow into an adjacent hole, or the hole itself may deteriorate or collapse.
Other anchoring methods form a hole at the base of the anchor pile and cement an enlarged portion of the pile into place. However, these steps are not necessarily easy to accomplish in the subsea environment and may be more complex than necessary to fill the job need.
Consequently, it is an object of this invention to develop an anchor that resists uplift loads and may be driven to an exact preset depth which may be less than the depth of refusal. It is another object of this invention to develop a pile which may be driven into soil of varying resistance and also be able to provide equal tension capacity for all of the tension leg platform tendons.